Jest tests for Netlify Functions

Have you struggled to figure out a good way to test your Netlify Functions? As you start to make more use of functions, you really need to have good unit tests in place for peace of mind during production deploys.

I was in the same position but after some research and trial and error, I'm happy that I have a good solution in place. In order to demonstrate how to set things up, I'll build upon my previous blog post that created a simple Netlify Function to print out "Hello world" followed by a random number.

At this point I'll assume you do not have Jest installed so I will work through the steps from the very beginning. Obviously, the first step is to install Jest!

yarn add jest --dev

You will also need to add additional config to package.json

"jest": {
  "moduleFileExtensions": [
  "moduleDirectories": [

and add to scripts

"test": "jest ./spec/javascripts"

That's all there is to get the basics setup and now we need a file to test your function which you can create at spec/javascripts/functions/hello_world.test.js. This is where it gets interesting because now we're at the point of figuring out how to test the function.

After some research I came across a package called lambda-tester which looked like it would fit the bill nicely. It's designed to simplify writing unit tests for AWS Lambda functions which of course is what Netlify Functions are under the hood. Install the package

yarn add lambda-tester --dev

and a simple test will look as follows

const LambdaTester = require("lambda-tester")
const myHandler = require("hello_world").handler

jest.spyOn(global.Math, "random").mockReturnValue(0.5);

describe("handler", function() {
  it("returns hello world followed by a random number", async function() {
    await LambdaTester(myHandler)
      .expectResolve((result) => {
          message: "Hello world 5",

If you run yarn test you will get an error stating that regeneratorRuntime is not defined! This can easily be fixed by adding another package

yarn add @babel/plugin-transform-runtime --dev

and add the following to your .babelrc file

"env": {
  "test": {
    "plugins": ["@babel/plugin-transform-runtime"]

which will allow you to use async/await in tests. Now yarn test should run successfully!

For this post I'll add in another quick example about how to easily handle Netlify environment variables in your function tests because that is likely to be a common use case. Instead of this line of code in your function

body: JSON.stringify({ message: `Hello world ${randomNumberGenerator()}` })

lets change the function code to look as follows

import { randomNumberGenerator } from "./utils/random_number_generator"
const { HELLO_WORLD } = process.env

export async function handler(event, context) {
  return {
    statusCode: 200,
    body: JSON.stringify({ message: `${HELLO_WORLD} ${randomNumberGenerator()}` })

and you can now add a key of HELLO_WORLD and value Hello world to your Netlify environment variables.

If you run your test now it will fail because ${HELLO_WORLD} is undefined. In order to fix this, add the following to your Jest config in package.json

"setupFiles": [

and create a new file at spec/javascripts/support/setEnvVars.js with the following content

process.env = {
  HELLO_WORLD: "Hello world"

Run your test again and everything should pass.

That's all there is to it and you can add yarn test to your Netlify build command to run the tests before each site build. Using lambda-tester makes unit testing your Netlify Functions a breeze and gives you peace of mind with every production deploy!